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Old 22nd November 2007, 05:45 PM   #21
Eva is offline Eva  Spain
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Oh, so you are asking me for the Ft of IRF540??? No, Ft is a bipolar-transistor thing meaningful only for linear amplifiers... MOSFETs in switching applications behave in another way, gate drive and package and circuit parasitistics are the limiting factors for switching speed.
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Old 22nd November 2007, 05:55 PM   #22
zox2003 is offline zox2003  United States
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Any examples or projects with PCB that you want to share with me and other people here?
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Old 22nd November 2007, 06:21 PM   #23
luka is offline luka  Slovenia
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Hi

why do you have that heatsink?? Kinda big, don't you think? I had my amp running at +/-30v, no heatsink, 4ohme load, pure sinus for some time with no problems. I don't have same transistors as you do, but still...
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Old 22nd November 2007, 06:25 PM   #24
zox2003 is offline zox2003  United States
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Luka,

Are you refering to SMD D class amplifier you have PCb for or some other amp?
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Old 22nd November 2007, 06:29 PM   #25
luka is offline luka  Slovenia
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Hi

Same amp, but that one is not smd and with very bad layout
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Old 22nd November 2007, 06:46 PM   #26
zox2003 is offline zox2003  United States
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Luka ,

did you work on some other amps beside one you mentioned?
what is your opinion on IR2104 as driver?

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Old 22nd November 2007, 06:52 PM   #27
luka is offline luka  Slovenia
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Hi

No that one is the only one so I can't tell you nothing on your IR, but it seems to me that most amps are made with IR2110/3 or IR2011
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Old 22nd November 2007, 06:58 PM   #28
zox2003 is offline zox2003  United States
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Luka,

Is there any way I can talk to you directly or do you have Yahoo Messenger so we can talk directly.I have some questions regarding that amplifier.
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Old 22nd November 2007, 07:03 PM   #29
luka is offline luka  Slovenia
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Hi

I have windows massenger... See my profile for acc
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Old 22nd November 2007, 09:11 PM   #30
Eva is offline Eva  Spain
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Rule 1: Dead time is the main cause of distortion in class D. Gate drive should have little (like 10ns) or no dead time (this is hard to achieve).

Rule 2: Keep switching supply currents confined in loops as small as possible and use SMD ceramic chip capacitors to get low supply impedance between 10Mhz and 100Mhz. Use PI filters to prevent the switching hash in the rails from propagating through PSU wiring.

Rule 3: Use RC snubbers to damp layout and transistor package parasitistics as required. Component Values have to be carefully chosen for maximum damping with minimum losses.

Rule 4: MOSFET gates have to be charged and discharged fast (and by fast I mean 20 to 200ns). Use the gate charge figures from MOSFET datasheets and the output current figures from driver datasheets to estimate gate charging times and choose a proper combination of MOSFETs and gate driver.

You have a lot of homework to do...

Just kidding...
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